1. Field of the Invention
The present invention relates to a device for attenuating unwanted waves in which can develop electron tubes having coaxial cylindrical electrodes.
This attenuation device is particularly advantageous in a grid tube in order to avoid the appearance of undesirable modes, especially the TE11 mode.
2. Discussion of the Background
Grid tubes are generally of the tetrode or triode type. A triode is represented diagrammatically in FIG. 1 to which reference is made. It includes cylindrical electrodes mounted coaxially around an axis of revolution XX'. The central electrode is the cathode 1 which emits electrons when it is heated. Around the cathode 1 is a control grid 2 and then an anode 3. In a tetrode there is an extra grid termed the screen grid placed between the control grid 2 and the anode 3.
Each inter-electrode space is associated with a resonant cavity 5 which consists of cylindrical walls 4 extending the electrodes and is limited heightwise by a movable plunger 6 allowing frequency adjustment. In FIG. 1, only the resonant cavity associated with the control grid 2--anode 3 inter-electrode space is represented completely for the sake of clarity.
Linkage between a respective electrode and a corresponding cavity wall 4 is effected by a respective cup-shaped conducting collar 7 fixed to the corresponding electrode and possibly a conducting ring 8 which includes elastic contacts and is inserted between the respective collar 7 and the corresponding wall 4.
Since the electrodes often have to a DC potential with respect to the cathode 1, it is advisable to insert a capacitor for HF short-circuiting and blocking of direct current between a ring 8 and a cylindrical wall for example.
The respective collars 7 are electrically insulated from one another by corresponding ceramic spacers 9 which also have the function of mechanical retention of the electrodes of the tube and a vacuum-tightness function. A difference of several thousand volts may exist between the control grid 2 and the anode 3 for example.
The inter-electrode space associated with its resonant cavity 5 forms a coaxial resonant circuit normally resonating in the TEM mode. The resonant circuit can however resonate in several other modes, some of which are particularly undesirable as for example the TE11 mode. This is the dominant mode of the TE modes in coaxial resonators and it has the lowest cutoff frequency. For a given resonator, the TM modes have much higher cutoff frequencies and present less of a problem. Indeed, the higher the frequency of a guided mode, the greater are the losses which it produces in the walls. The waves corresponding to modes whose cutoff frequency is high weaken rapidly and cannot propagate.
In a grid electron tube operating as an amplifier, a very-high-frequency signal is injected in the region of the cavity of the input resonant circuit situated between the cathode 1 and the control grid 2 and the signal is extracted after amplification in the region of the cavity of the output circuit situated between the control grid 2 and the anode 3.
Under certain conditions, unwanted resonances in undesirable modes set up in the output resonant circuit may excite the input resonant circuit and give rise to sustained oscillations.
Several means are known for avoiding sustained oscillations. These means consist in increasing the losses from the output resonant circuit for the undesirable frequencies.
A first known solution consists in coupling waveguides each terminating in an absorber to the output resonant circuit. These waveguides are stationed radially around the resonant cavity. The dimensions of the guides are chosen so as not to influence the TEM mode. These guides have a cutoff frequency which lies between the frequency of the undesirable modes and the range of frequencies of the amplifier. The opening of these guides forms, in the outer wall of the resonant cavity, series inductances which are traversed by the surface currents generated by the TEM mode. As a consequence the frequency range of the amplifier is lowered.
Moreover, the lower the freqency of the modes to be eliminated, the greater is the cross-section of the guides. There may be difficulties in accommodating several guides at the periphery of the resonant cavity. In any event, even if guides can be accommodated they considerably increase the bulkiness of the base of the tube. It has been proposed to bend them in order to reduce the bulkiness but this solution is expensive to achieve and not truly satisfactory for reducing bulkiness.
Another known solution consists in stationing above the control grid or the screen grid, in the bottom of the anode a cylindrical conducting structure including several resonant circuits of the distributed-constants RLC type tuned to the frequency of the unwanted waves to be attenuated. The material of the structure is chosen so that it exhibits high losses. This solution has the drawback of being selective: the conducting structure operates only within a narrow band since it includes resonant circuits. Given the small amount of room available in the anode, it is difficult, even impossible, to accommodate other conducting structures whose resonant circuits would be tuned to other frequencies to be eliminated.
Another known solution consists in stationing, in the output cavity, ferrites exhibiting high losses for the frequencies of the unwanted waves. However, choosing and positioning the ferrites is difficult. Futhermore, their effectiveness is not necessarily assured in all operating cases.
Accordingly, an object of the present invention is to remedy these drawbacks. To accomplish this, it proposes a device for attenuating unwanted waves for an electron tube with coaxial electrodes which is simple to make and hence cheap, which is effective over a plurality of frequencies and which does not modify the bulkiness of the tube.